Application information

ST1S31

18/35

DocID022998 Rev 4

Equation 22

So if the inductor value decreases, the peak current (which must be lower than the current

limit of the device) increases. The higher the inductor value, the higher the average output

current that can be delivered, without reaching the current limit.

In Table 7 some inductor part numbers are listed.

5.3

Output capacitor selection

The current in the output capacitor has a triangular waveform which generates a voltage

ripple across it. This ripple is due to the capacitive component (charge or discharge of the

output capacitor) and the resistive component (due to the voltage drop across its ESR). So

the output capacitor must be selected in order to have a voltage ripple compliant with the

application requirements.

The amount of the voltage ripple can be calculated starting from the current ripple obtained

by the inductor selection.

Equation 23

For a ceramic (MLCC) capacitor, the capacitive component of the ripple dominates the

resistive one. While for an electrolytic capacitor the opposite is true.

As the compensation network is internal, the output capacitor should be selected in order to

have a proper phase margin and then a stable control loop.

The equations of Section 5.2 help to check loop stability given the application conditions,

the value of the inductor and of the output capacitor.

Table 7. Inductors

Manufacturer

Series

Inductor value (

Saturation current (A)

Coilcraft

XAL50xx

1.2 to 3.3

6.3 to 9

XAL60xx

2.2 to 5.6

7.4 to 11

MSS1048

1.0 to 3.8

6.5 to 11

Würth

WE-HCI 7030

1.5 to 4.7

7 to 14

WE-PD type L

1.5 to 3.5

6.4 to 10

Coiltronics

DR73

1.0 to 2.2

5.5 to7.9

DR74

1.5 to 3.3

5.4 to 8.35



I

L

2

--------

+

=

V

OUT

ESR

I

MAX



I

MAX



-------------------------------------

+

=